As for the "Realism" for any of the mods well that is always debateable as since to my knowlege none of our moders (im not knocking any of our moders btw just stating fact) actualy served aboard a uboat in ww2, or blew up a ship with a torpedo that has similar damage potential to the ones the germans used. So they are all founded on that modder's opinion on what is "realistic" (and what is possible to mod in the game).
In the end i suggest trying the 2 main super mods (Grey Wolves and NYGM) and decide for yourself which feels right to you
Oh ya and welcome to the forums.
there are lots of guides and other helpfull stuff in this forum, i suggest you give them a good searching for tips etc.